Support New Rack APIs

This changes the underlying API from using the legacy Rack::Session::Abstract::ID to Rack::Session::Abstract::Persisted, in order to bring it in line with the abstract cache stores provided by Rails that make use of this gem as a backend. As a consequence, this also drops support for Rack 2, so this prerelease is designed to give it a shot in the wild without affecting too many machines, we'll release a full v2.1.0 update in the coming weeks.